We make a Meyer lemon curd at work which we are having problems with. My boss buys Meyers by the case and squeezes and freezes the juice. When we need to make the curd, we thaw the juice and then proceed to make the curd. Well, for some strange reason after the curd is cooked and chilled, it never firms up. It's totally runny. We cook it until the whisk leaves tracks. I'm suspecting it's the juice(because we make another curd with regular fresh lemons and have not had the problem)---or at least the freezing/thawing and sometimes re-freezing. Any thoughts?